Methods for producing metal oxide particles by sol-gel methods are commonly known. For example, silica flakes are produced by the steps of: 1) hydrolyzing and condensation-polymerizing a silicon alkoxide in an alcohol aqueous solution to form a silica sol; 2) applying the silica sol onto a substrate to form a film; and 3) peeling the film from the substrate. The use of an alkaline silica sol results in a low strength of the film, leading to reduction in production yield. Therefore, an acidic silica sol is suitable for the above method.
The above silica flakes are called glass flakes, and are used by being dispersed in a matrix formed of another material. For example, the strength and dimensional accuracy of a resin molded body can be improved by adding glass flakes. There are also known bright pigments whose reflectivity has been increased by covering the surfaces of glass flakes with a film of metal or metal oxide. Bright pigments are blended with cosmetics, inks, etc., to enhance the commercial values.
For example, Patent Literature 1 to 3 disclose the details of methods for producing glass flakes using sol-gel methods.
Silica particles in other forms than flakes also play important roles in some uses. For example, spherical silica is used as spacers for liquid crystal display panels and touch panels. Spherical silica having uniform particle diameters and suitable for use as spacers is produced by mixing a silicon alkoxide, an alcohol, and aqueous ammonia at a temperature around room temperature. In this method, which is called a “stober method”, very fine silica cores grow with progress of hydrolysis and dehydration condensation, and spherical silica having uniform particle diameters is formed.
Silica particles account for the highest proportion among metal oxide particles mass-produced actually by sol-gel methods. However, sol-gel methods can also be carried out using a metal alkoxide containing another metal element such as titanium and zirconium. Titanium oxide fine particles obtained by sol-gel methods have been attracting attention because of their ultraviolet-shielding function and photocatalytic function.
In addition, it has been attempted to produce particles having various properties by applying sol-gel methods. For example, it has been proposed to produce silica particles having so-called mesopores (mesoporous silica) by a sol-gel method using a surfactant as a template. The mesoporous silica is useful as an adsorbing material or the like.